The Grand Slam was a 22,000 lb (10,000 kg) earthquake bomb used by RAF Bomber Command against strategic targets during the Second World War.Known officially as the Bomb, Medium Capacity, 22,000 lb, it was a scaled-up version of the Tallboy bomb and closer to the original size that the bombs' inventor, Barnes Wallis, had envisaged when he first developed his earthquake bomb idea. It was also nicknamed "Ten ton Tess""When the success of the Tallboybomb was proved, Wallis designed a yet more powerful weapon… This 22,000 lb. bomb did not reach us before the spring of 1945, when we used it with great effect against viaducts or railways leading to the Ruhr and also against several U-boat shelters. If it had been necessary, it would have been used against underground factories, and preparations for attacking some of these were well advanced when the war ended."

— Sir Arthur Travers Harris (1947)

On 18 July 1943, work started on a larger version of the Tallboy bomb, which became the Grand Slam. As with the original Tallboy, the Grand Slam's fins generated a stabilizing spin and the bomb had a thicker case than a conventional bomb, which allowed deeper penetration. After the hot molten Torpex was poured into the casing, the explosive took a month to cool and set. Like the Tallboy, because of the low rate of production and consequent high value of each bomb, aircrews were told to land with their unused bombs on board rather than jettison them into the sea if a sortie was aborted.After release from the Avro Lancaster B.Mk 1 (Special) bomber,[2] the Grand Slam would reach near-supersonic speed, approaching 1,049 ft/s (715 mph) - (320 m/s, 1150 km/h). When it hit, it would penetrate deep underground before detonating. The resulting explosion could cause the formation of a camouflet (cavern) and shift the ground to undermine a target's foundation.Unlike the Tallboy, the Grand Slam was designed to penetrate concrete roofs; it was not prone to premature detonation or disintegration.[10] This made the weapon far more effective against hardened targets than any existing bomb.The first Grand Slam was tested at the Ashley Range in the New Forest, on 13 March, 1945.

A 617 Sqn Lancaster dropping a Grand Slam bomb on the Arnsberg viaduct, March 1945.

The damage caused by one of the two Grand Slams that
penetrated the Valentin submarine pen, 27 March 1945; a figure stands at
the edge of the rubble pile, providing a sense of scale to the damageBy the end of the war, 42 Grand Slams had been dropped on active service.Bielefeld, 14 March 1945

The
No. 617 Squadron RAF Avro Lancaster of Squadron Leader CC Calder
dropped the first Grand Slam bomb from 11,965 ft (3,647 m) on the
Schildesche viaduct.[14] More than 100 yards of the Bielefeld viaduct
collapsed through the earthquake bomb effect[15] of the Grand Slam and
Tallboy bombs of No. 617 Squadron. No aircraft were lost.

Arnsberg, 15 March 1945Two
aircraft of No. 617 Squadron RAF each carried a Grand Slam and 14
aircraft of No. 9 Squadron RAF carried Tallboy bombs to attack the
railway viaduct in poor weather. One Grand Slam and 10 Tallboys were
dropped, while one of the Lancasters was forced to bring its bomb back.
The viaduct was not cut and no aircraft were lost.

Arnsberg, 19 March 194519
Lancasters of No. 617 Squadron, six carrying Grand Slams, the remainder
Tallboys, attacked the railway viaduct at Arnsberg. All Grand Slams
were dropped and blew a 40-foot (12 m) gap in the viaduct. The standing
structure was severely damaged.

Arbergen, 21 March 194520
Lancasters of No. 617 Squadron, two carrying Grand Slams, the remainder
Tallboys, attacked the railway bridge at Arbergen. The Grand Slams
landed off target due to heavy flak and aiming problems; 2 Tallboy hits
caused sufficient damage to the approaches to the bridge to put it out
of use. One 617 Lancaster was lost.

Nienburg, 22 March 194520
Lancasters of No. 617 Squadron, six carrying Grand Slams, the remainder
Tallboys, attacked the railway bridge at Nienburg, between Bremen and
Hanover. 5 Grand Slams made direct hits and the bridge was completely
destroyed. Another 5 bombs were brought home by the squadron.Bremen, 23 March 1945

20
Lancasters of No. 617 Squadron, six carrying Grand Slams, the remainder
Tallboys, attacked a railway bridge near Bremen. The Grand Slams appear
to have landed too far from the target, which was brought down by a
Tallboy. Author Jon Lake claims instead that two Grand Slams struck the bridge.[18]Farge, 27 March 1945

20
Lancasters of No. 617 Squadron attacked the Valentin submarine pens,[4]
a huge, nearly-ready structure with a concrete roof up to 23 ft (7.2 m)
thick. Two Grand Slam bombs penetrated in parts of the pen with a 14 ft
5 inches (4.5 m) thick roof,[4][23] which rendered the shelter
unusable. No aircraft were lost.

Hamburg, 9 April 194517
aircraft of No. 617 Squadron, two with Grand Slams and the remainder
with Tallboy bombs successfully attacked the U-boat shelters. The Grand
Slams appear to have missed, but six Tallboy hits caused considerable
damage. No aircraft were lost.

Heligoland, 19 April 194520
aircraft of No. 617 Squadron, six with Grand Slams and the remainder
with Tallboy bombs, along with 16 aircraft from No. 9 Squadron, attacked
coastal gun-batteries. No aircraft were lost.

Post–war operationsBeginning
in March 1946, Project Ruby was a joint, Anglo–American project to
investigate the use of penetration bombs against heavily–protected,
concrete targets. The target selected was the Valentin submarine pens,
that had been rendered unusable and abandoned since 617 Squadron's
attack on 27 March 1945. Grand Slams were carried by Lancasters from No.
15 Squadron RAF and US Boeing B-29 Superfortress. Around 140 sorties
were flown, testing a range of different bombs.

A Grand Slam bomb at the RAF Museum, LondonFive
complete Grand Slam bombs are preserved and displayed in the United
Kingdom at the RAF Museum, London; Brooklands Museum; RAF Lossiemouth;
Dumfries and Galloway Aviation Museum and the Battle of Britain Memorial
Flight Visitors' Centre at RAF Coningsby. The main portion of a bomb,
without the lightweight tail, is at the Kelham Island Museum in
Sheffield.

A T-14 bomb (an American-made variant of the Grand Slam) is displayed at the Air Force Armament Museum in the United States.